On an early morning on May 24, 1987, Kenneth Parks drove 20 kilometers from Pickering, Ontario to the house of his in-laws in Scarborough, Ontario. He entered their house with a key they had previously given him and used a tire iron to bludgeon his mother-in-law to death. He then turned on his father-in-law, attempting to choke him to death, but he managed to survive the attack. He got back in his car and, despite being covered with blood, drove straight to a nearby police station and confessed, turning himself in, stating "I think I have just killed two people".[1]

At trial, Parks argued that he was automatistic and not criminally liable. In his defence, a doctor testified as to his mental state at the time of the murder. From the doctor's evidence, it was determined that the accused was sleepwalking at the time of the incident, and that he was suffering from a disorder of sleep rather than neurological, psychiatric or other illness. Five neurological experts also confirmed that he was sleepwalking during the time of the incident. The jury acquitted Parks.

The issue before the Supreme Court was whether the condition of sleepwalking can be classified as non-insane automatism or should it be classified as "disease of the mind" (i.e. mental disorder automatism) and warrant a verdict of "not guilty for reason of insanity". This distinction is a matter of law and decided by the judge.

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The court upheld the acquittal as the evidence presented a reasonable doubt that Parks acted voluntarily.
Chief Justice Antonio Lamer held that the trial judge was correct in his analysis of the evidence and his decision not to characterize sleepwalking as a mental disorder.

La Forest, writing for L'Heureux-Dubé and Gonthier JJ., agreed with Lamer in the characterization of the evidence, but looked further into the public policy of the defence. La Forest noted that the defence of mental disorder provides for a criminal exception which must be weighed against the interest in public safety. The applicability of the defence must focus on the likelihood of recurrence. For a person to be exempt from criminal liability under the "disease of the mind" defence they must be a "continuing danger" to the public and the condition must be an "internal cause" that stems from the accused's emotional or psychological state.